Windows & iPhone themes are everywhere. Google Blackberry Themes to find a ton of sites.
You can also start of at a place like www.pimpmyberry.com for themes. If a theme says Premium then its going to cost you money, otherwise it is probably free.
A word of advice. Make sure you are downloading a theme for your specific phone model AND make sure that the theme you are downloading works on your operating system. If you are still on OS version 4.2 or 4.3 do not try installing a theme that says 4.5 in the msg thread.
Great IRC channel:
join #blackberry on EFNet today!